Logan Steele
The ciliary muscle is an intrinsic muscle of the eye that controls the movement of the lens and the pupil, allowing us to see objects clearly at varying distances. It is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary muscle is an important component of the ciliary body, which is an inner eye structure that forms a semi-transparent ring on the outer surface of the choroid. The ciliary body is responsible for producing the aqueous humour, a fluid that nourishes the lens and cornea. The ciliary muscle controls the shape of the lens by contracting and relaxing, which changes the thickness and curvature of the lens, enabling the eye to focus at different distances.
| Characteristics | Values |
|---|---|
| Definition | An intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer) |
| Function | Controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into Schlemm's canal |
| Controlled By | The oculomotor nerve |
| Location | Behind the iris of the eye |
| Composition | Smooth muscle fibers oriented in three different directions: longitudinal, radial and circular |
| Blood Supply | The central artery of the retina, short and long posterior ciliary arteries, and the anterior ciliary arteries |
| Associated Conditions | Open-angle glaucoma (OAG), closed-angle glaucoma (CAG), uveitis, iridocyclitis |
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What You'll Learn
The ciliary muscle is an intrinsic muscle of the eye
The ciliary muscle changes the shape of the lens within the eye, but not the size of the pupil, which is carried out by the sphincter pupillae muscle and dilator pupillae. The ciliary muscle, along with the pupillary sphincter muscle and pupillary dilator muscle, is sometimes referred to as intrinsic ocular muscles or intraocular muscles. The ciliary muscle develops from mesenchyme within the choroid and is considered a cranial neural crest derivative.
The ciliary muscle receives parasympathetic fibres from the short ciliary nerves that arise from the ciliary ganglion. The parasympathetic postganglionic fibres are part of cranial nerve V1 (Nasociliary nerve of the trigeminal). The ciliary muscle also receives blood vessels from the long posterior ciliary arteries and the major iris circle. Blood flow through the ciliary body is about 7% of the total ocular flow.
The ciliary muscle is elongated and triangular in shape, located beneath the anterior sclera just posterior to the limbus. The shortest side of the triangular region faces anterior-inward, with the base of the iris inserted into this region. The ciliary muscle occupies the biggest portion of the anterior aspect of the ciliary body, with the ciliary processes lying posterior to it.
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It controls the eye's ability to view objects at varying distances
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). It is an important part of the eye that controls the eye's ability to view objects clearly at varying distances.
The ciliary muscle controls the shape of the lens, while the ciliary epithelium regulates aqueous humor production. The ciliary body is a 5–6 mm wide ring of tissue extending from the scleral spur anteriorly to the ora serrata posteriorly and is the anterior continuation of the choroid. It can be further divided into the pars plicata and pars plana. The ciliary muscle is suspended by a fine meshwork of zonular fibres. These are fine elastic fibres inserted all around the lens equatorial region into the lens capsule, with no clear delineation into separate regional groups.
The ciliary muscle controls the eye's ability to view objects at varying distances by changing the shape of the lens within the eye. When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye. This releases the tension on the lens caused by the zonular fibres (fibers that hold or flatten the lens). This release of tension of the zonular fibres causes the lens to become more spherical, adapting to short-range focus. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance, improving long-range focus.
The ciliary muscle is an important part of the eye that contributes to a person's ability to view objects clearly at varying distances.
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It changes the shape of the lens, not the size of the pupil
The ciliary muscle is an intrinsic muscle of the eye that controls the movements of the lens and pupil. It is formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). The ciliary body, which contains the ciliary muscle, is a 5–6 mm wide ring of tissue extending from the scleral spur anteriorly to the ora serrata posteriorly. It is the anterior continuation of the choroid.
The ciliary muscle is responsible for changing the shape of the lens, not the size of the pupil. The pupil size is controlled by the sphincter pupillae muscle and dilator pupillae. The ciliary muscle, along with the sphincter pupillae and dilator pupillae muscles, are sometimes referred to as intrinsic ocular muscles or intraocular muscles.
The ciliary muscle controls the shape of the lens by contracting and relaxing. When the ciliary muscle contracts, it moves forward, releasing the tension on the lens caused by the zonular fibres. This allows the lens to become more spherical due to its elasticity, enabling short-range focus. Conversely, when the ciliary muscle relaxes, the zonular fibres become taut, flattening the lens and increasing the focal distance for long-range focus.
The process of changing the shape of the lens to focus on objects at varying distances is called accommodation. The ciliary muscle is, therefore, essential for clear vision and plays a crucial role in a person's ability to view objects clearly at different distances.
The ciliary body also has another important function: it produces the aqueous humour, a fluid in the eye that nourishes the lens and cornea.
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Glaucoma can be treated by targeting the ciliary muscle
The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). It controls the shape of the lens, allowing the eye to focus on objects at varying distances. The ciliary muscle is an important part of the eye that enables clear vision.
Glaucoma is a chronic, progressive, and irreversible optic neuropathy that can lead to blindness. It is the second most common cause of blindness worldwide, affecting approximately 80 million people. Glaucoma is characterised by high intraocular pressure, which often causes damage to the optic nerve. The ciliary body produces aqueous humour, a fluid in the eye, and is the main target of many medications against glaucoma. Inhibition of the ciliary body leads to a reduction in aqueous humour production and a subsequent drop in intraocular pressure.
Open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) can be treated by targeting the ciliary muscle with muscarinic receptor agonists such as pilocarpine. These agonists cause rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork and facilitating drainage of the aqueous humour into the canal of Schlemm, ultimately decreasing intraocular pressure. This treatment approach aims to alleviate the increased intraocular pressure observed in glaucoma by enhancing fluid drainage and reducing fluid build-up in the eye.
In addition to pharmacological interventions, cyclodestruction is another treatment modality for glaucoma that targets the ciliary muscle. Cyclodestruction involves the coagulation or destruction of the ciliary body to reduce aqueous humour production. While early attempts at cyclodestruction carried a risk of damaging surrounding tissues, newer modalities like endoscopic cyclophotocoagulation (ECP) offer a more targeted and controlled approach, minimising complications. The development of targeted ciliary body destruction techniques has expanded the role of cyclodestruction in glaucoma treatment, making it a valuable option in managing this sight-threatening condition.
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The ciliary body is a component of the uvea
The ciliary body is a circular structure, approximately 5-6 mm wide, and it contains the ciliary muscle. The ciliary muscle is a smooth muscle that forms a ring in the eye's middle layer, the uvea. It controls the accommodation of the lens, allowing the eye to focus on objects at different distances. The contraction and relaxation of the ciliary muscle change the thickness and curvature of the lens, enabling the eye to adjust its power and focus. This process is essential for clear vision at varying distances.
The ciliary muscle also regulates the flow of aqueous humour, a fluid produced by the ciliary body and the ciliary epithelium. Aqueous humour is important for maintaining the shape and pressure of the eye. Additionally, the ciliary body provides attachment sites for the zonular fibres, which help to maintain the position of the lens within the eye. These zonular fibres are fine elastic fibres that form a meshwork around the lens, connecting it to the ciliary body.
The ciliary body, as part of the uvea, can be affected by various conditions and diseases. Uveitis, for example, is a term used to describe inflammation and possible infection of the uvea. If left untreated, uveitis can lead to vision loss. Another condition, iridocyclitis, involves inflammation of the iris and the ciliary body, which can also impact vision. Tumours, masses, and cysts can also affect the ciliary body and the uvea. Intraocular melanoma, for instance, commonly arises in the uvea and can progress into the ciliary body.
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Frequently asked questions
Yes, the ciliary muscle is an intrinsic muscle of the eye.
The ciliary muscle is a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer).
The ciliary muscle controls the shape of the lens, allowing the eye to focus on objects at varying distances.
The ciliary muscle contracts and relaxes, which changes the thickness and curvature of the lens, allowing the eye to change power and focus at different distances.
The ciliary body is an inner eye structure that forms a semi-transparent ring on the outer surface of the choroid. It includes the ciliary muscle and the finger-like ciliary processes.
